Recently, the development of next-generation clean energy has become more important due to severe environmental pollution and depletion of fossil fuels. Particularly, it is expected that solar cells, which are devices capable of converting solar energy directly into electrical energy, will become an energy source capable of solving future energy problems since they generate little air pollution, their resources are inexhaustible and they have semi-permanent life time.
Such solar cells are classified into various types depending on a material used as a light absorbing layer. Silicon solar cells using silicon are most commonly used presently. However, the price of silicon has soared due to a recent deficient supply of silicon. Accordingly, there is a growing interest in thin-film solar cells. Since the thin-film solar cells are manufactured to have a thin thickness, the thin-film solar cells require less consumption of materials and are light, and, thus, have a wide range of applications. Studies on amorphous silicon, CdTe, and CIS (CuInSe2, CuIn1-xGaxSe2, CuIn1-xGaxS2, etc.) have been actively conducted as materials of such thin-film solar cells. A CIS-based thin film is one of I-III-IV compound semiconductors, and particularly, a CIGS solar cell has the highest conversion efficiency (about 22.3%) out of thin-film solar cells prepared experimentally. However, in used therein is a rare element found in a relatively small amount of its resource, and the price thereof is on an upward trend due to the demand for ITO used in the display industry, which may constitute an obstacle to mass production. In order to solve this problem, compound semiconductors such as Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) have been actively researched to be used for the development of a low-price solar cell as alternatives to CIGS-based thin film materials by substituting abundant elements Zn and Sn for rare elements of In and Ga. The compound semiconductors are advantageous in preparing a high-efficiency solar cell suitable for the solar spectrum and expected simultaneously to solve other problems of the CIGS-based solar cells.
Meanwhile, Korean Patent No. 10-1339874 discloses a method for preparing a double-grading CZTS-based thin film.